1. Field of the Invention
The present invention relates to a laser treatment apparatus for performing treatment by irradiating a part to be treated (an affected part) with a treatment laser beam.
2. Description of Related Art
Laser treatment apparatuses for a photocoagulation treatment include an apparatus of a parfocal type constructed to form an image of an exit end face of an optical fiber which guides a treatment laser beam (hereinafter, a treatment beam) emitted from a laser light source on a part to be treated (hereinafter, a treatment part), thereby irradiating the treatment part with the treatment beam. When an intensity distribution (i.e., an energy distribution) of the treatment beam at the exit end face of the optical fiber is uniform, this parfocal type apparatus provides a uniform intensity distribution of the treatment beam on an irradiation plane of the treatment part. However, in a retinal photocoagulation treatment, a tissue irradiated does not evenly react even where the intensity distribution at the irradiation plane is uniform, so that a center portion of an irradiation spot tends to first begin to burn (coagulate). In particular, when the size of the irradiation spot of the treatment beam is set at, for example, as large as 500 μm in diameter, the intensity distribution in the irradiation spot has an influence on a coagulation effect. Accordingly, an ideal intensity distribution in a large-sized irradiation spot of the treatment beam has been considered to be high (strong) in a peripheral portion and low (weak) in a center portion. On the other hand, in an iris incision treatment (iridotomy), it is an ideal to set the size of the irradiation spot of the treatment beam at, for example, as small as 50 μm in diameter, thereby increasing an energy density in the irradiation spot.
For example, Japanese patent unexamined publication No. 2001-8945 discloses such apparatus structured to increase or decrease the intensity distribution in the center portion of the irradiation spot as compared with that in the peripheral portion. This apparatus is provided with a filter capable of changing a beam intensity distribution. To change the intensity distribution in the irradiation spot, this filter is attached to a rotatable support board and placed adjacent to the exit end face of the optical fiber. Alternatively, the filter is placed in a position unconjugated with the irradiation plane and moved in an optical axis direction.
In the above conventional apparatus, however, the support board of the beam intensity distribution changing filter placed adjacent to the exit end face of the optical fiber has to be rotated in order to produce different intensity distributions according to spot sizes, large and small. This would lead to a complicated structure, which needs a complex operational control. In this structure, it is uneasy to align the center of the filer with the optical axis during adjustment in each case. The exit end face of the optical fiber and the irradiation plane are positioned to be conjugated with each other, so that slight misalignment of the optical axis would affect the intensity distribution on the irradiation plane. Since the exit end face of the optical fiber is several tens μm in diameter, a light-shielding part of the filter needs to be made very small and therefore it is hard to manufacture.
In the case where the beam intensity distribution changing filter is inserted in a position unconjugated with the irradiation plane, a structure of some kind must be provided to move the filter in the optical axis direction in order to change the intensity distribution. The apparatus structure would further become complicated.